Celtics assistant Lawrence Frank is among three finalists for the Rockets’ head coaching job, according to the Houston Chronicle. “I know Lawrence is interviewing for head coaching opportunities, and he should be hired as a head coach,’’ Ainge said. “My guess is that he’ll be hired as a head coach somewhere.’’ Celtics great Kevin McHale and Mavericks assistant Dwane Casey are the other finalists, according to the report.